If there are mirror particles under an unbroken symmetry \(G^{\prime }\) , which is a copy of the Standard Model gauge group \(G=SU(3) \times SU(2) \times U(1)\) , the microphysics of the mirror and the Standard Model sector is identical. Although the renormalizable gauge kinetic mixing and Higgs portal interactions are allowed between the two sectors, these couplings could be sufficiently negligible in the early universe [1]}. Thus Standard Model particles interact with their mirror partners only gravitationally, and the mirror baryons \(b^{\prime }\)  may be suitable candidates for DM. The prime denotes the mirror sector.
